Field
Embodiments of the disclosure relate to separations for components of a natural gas stream. In particular, embodiments of the disclosure relate to producing substantially pure methane from natural gas using pressure swing adsorption (PSA). In addition, the disclosure relates to recovery of heavier hydrocarbons, such as ethane, propane and butane from a natural gas stream, and separations of hydrocarbon gas streams comprised mostly of hydrocarbons heavier than methane.
Description of the Related Art
Raw natural gas contains concentrations of natural gas liquids (NGLs) and other non-methane contaminants that need to be removed by gas processing in order to meet specifications required by a pipeline or end use. As well, NGL components such as ethane, propane, and butane can have higher sales values than pipeline gas, which is largely comprised of methane. Ethane is a valuable chemical feedstock, and propane and butane can be blended to form liquefied petroleum gas (LPG) which is a valuable residential fuel. Therefore, NGLs are oftentimes extracted and fractionated in gas processing plants in accordance with the specific requirements of the regional markets and customers. Generally, commercial NGL specifications require less than about 0.5% by liquid volume of methane and less than 500 ppm of CO2 by volume in liquid.
As a commercial fuel source, natural gas distributors and consumers need to know the expected range of quality of the fuel being delivered, and ideally need to have some control over the variability of that fuel to assure compliance with regulations, to protect equipment, and most importantly, to ensure safety for all involved in natural gas processing, transport, and use. Therefore, specifications help limit the range of variability inherent in natural gas transported around the world. Generally, pipeline specifications indicate that there should be less than about 4.0 mol. % of other non-hydrocarbon gases (for example N2+CO2) in the natural gas, less than about 10 mol. % ethane in the natural gas, and the specific energy content of the natural gas should not exceed about 1,100 British thermal unit per standard cubic foot of natural gas (btu/scf).
Variables that affect choice of the most cost-effective process for maximizing NGL separation and recovery include: inlet conditions such as for example gas pressure, richness and contaminants; downstream conditions such as for example residue gas pressure, liquid products desired, and liquid fractionation infrastructure; and overall conditions such as for example utility costs and fuel value, location, existing location infrastructure and market stability. Because of this variability, there are a number of ways to recover NGLs from natural gas streams, and market demand and perceived return on investment drive the technology choice.
Mechanical refrigeration is a conventional option for NGL recovery, where natural gas is chilled until heavy components such as hexanes and heavier hydrocarbons (C≥6 hydrocarbons) condense out of the feed gas. Some of the intermediate components, such as butane and pentane can also be recovered, but there is limited recovery of ethane and propane. In order to achieve better recovery of ethane and propane from feed gases, cryogenic or turboexpander processes are typically used. These ‘cryo’ processes use the expansion of the natural gas stream to reduce the temperature to about −120° F. to about −140° F., so that most of the natural gas becomes liquefied and can be separated using distillation columns. This technology offers improved NGL recovery potential, but at much higher capital and operating expenditures. Cryo processes also require longer lead times to build and fabricate the specialty equipment necessary for their operation, such as the turboexpanders and aluminum heat exchangers.
Expander-based cryogenic processes require high inlet pressures to produce desired distillation column top temperatures for achieving optimal ethane and propane recovery. In most instances, an inlet pressure of greater than about 800 psia is desired for expander processes, meaning that low pressure gases must require significant inlet compression for separation to be efficient. Economies of scale then dictate that large cryogenic trains are necessary to share the “per unit cost” of compression, both at the inlet and to bring sales gas back to suitable pipeline pressures. These large trains are less tolerant of turndown because with reduced flow, either the turboexpander will not be able to achieve the low temperatures needed to operate the distillation/demethanizer column or the flowrates in the demethanizer will be insufficient to maintain the proper flow patterns.
Carbon dioxide in a feed gas will normally be split between the heavier hydrocarbons and methane, potentially affecting the product specifications, both for heavy and light products. Carbon dioxide can also freeze in a cryogenic or refrigeration process. Any ‘cold’ process for recovery of heavier hydrocarbons when carbon dioxide is present in the inlet gas must either operate in a region that will avoid freezing or provide carbon dioxide removal from one or more streams. Separation with greater than about 2 mol. % carbon dioxide in the inlet gas is not possible with cryogenic processes because freezing will result at either the top of the demethanizer column or at the side of the reboiler. This means that a cryogenic ethane recovery facility will require treating of carbon dioxide at more than one location of the process.
As richness of an inlet gas increases, heat exchanger pinch points will begin to appear in a cryogenic process. An external refrigeration system will be required to complement the cryogenic process to avoid these pinch points and to provide the energy to compensate for the relatively large amounts of energy leaving the system as liquid NGL product. Specifically, this occurs when the process is targeted to enhance recovery of ethane from a raw natural gas stream. As ethane recovery percentage increases, energy intensity also increases significantly. A typical ethane recovery range can be between about 60-85% for a cryogenic process, and any greater percentage of ethane recovery becomes more difficult and energy intensive because of the significant recompression horsepower required to enhance ethane recovery processes. Ethane recovery using mechanical refrigeration is not practical for industrial application.
The presence of large amounts of light inert gases also impacts ethane recovery in a cryogenic plant because the light components interfere with the efficiency and the ability to condense the reflux stream within the cryogenic process.
Undesirably, in existing processes, unrecovered propane and butane in a sales gas stream, which are more valuable as liquid products, will be sold at a discount in the sales gas (methane). In addition, unrecovered propane and butane will result in an increase in heating value and dewpoint of the sales gas, potentially exceeding pipeline specifications and resulting in financial penalties.